<<...One possible explanation is that interstellar travel is just too <A HREF="http://www.space.com./sciencefiction/larryniven/niven_making_somebody_pay_000914.html">costly</A>.
Consider how expensive it would be for us to populate another star system.
Imagine sending a small rocket to Alpha Centauri, one that’s the size of the
Mayflower (180 tons, with 102 pilgrims on board). Your intention is to get
this modest interstellar ark to our nearest stellar neighbor in 50 years,
which requires about 150 billion billion joules of energy.

No one’s sure what aliens pay for energy, but here on Earth the going rate is
about ten cents a kilowatt-hour. So the transportation bill per pilgrim would
be $40 billion. That’s a lot of moolah, a lot more than it takes to buy each
emigrant a few thousand six-bedroom palaces and set him up for life. The fact
that the trip is costly, in whatever currency, is reason enough to deter any
alien society from trying to settle distant real estate. With far less
expenditure, the extraterrestrials could pursue the good life <A HREF="http://ww.space.com/news/milan_visions_991117.html">at home</A>...>>